The present invention relates to a nonvolatile memory, and particularly to a structure of a memory cell and a manufacturing method thereof.
Related arts will be explained using the following cited references:    Patent document 1: Japanese Unexamined Patent Publication No. 2001-168219    Nonpatent document 1: Yutaka Hayashi, et al, “Twin MONOS Cell with Dual Control Gates”, 2000 Symposium VLSI Technology Digest of Technical Papers.
FIG. 2 is a configuration diagram of a conventional nonvolatile memory described in the nonpatent document 1 and shows a cross-section of a twin MONOS (Twin Metal-Oxide-Nitride-Oxide-Semiconductor) cell having two control gates separated from each other. One memory cell MC comprises two memory elements ML and MR, one diffusion region D (consisting of diffusion regions D1 and D2) and one word gate WG. The memory elements ML and MR respectively comprise CGL and CGR each equivalent to half of a control gate CG formed of polysilicon, and insulating films, i.e., ONO (Oxide-Nitride-Oxide) films constituted of an oxide film, a nitride film and an oxide film, which are disposed below the CGL and CGR to store electrical charges therein. The pair of memory elements ML and MR is disposed on both sides of the common control gate CG and shares the use of adjacent memory cells MCs and a word gate WG.
The control gates CGL and CGR divided into the two are electrically connected and share the same diffusion regions D which serve as bit lines. However, the ONO films disposed on the sides below the control gates CGL and CGR are separated from each other. The word gates WG, control gates CG and diffusion regions D are electrically connected in a memory array. For example, control lines are respectively provided on the diffusion regions D so as to be parallel with the diffusion regions and orthogonal to word lines.
Write and read operations of such a twin MONOS cell are performed as follows:
When, for example, writing (injection of electrons) is effected on the memory element MR, the potential of the diffusion region D1 on the memory element ML side is set to 0V, and the potential of the diffusion region D2 on the memory element MR side is set to 5V. The potential of the control gate CGL on the memory element ML side is set to 3.3V, and the potential of the control gate CGR on the memory element MR side is set to 6V. Further, the potential of the word gate WG is set to 1.5V. Incidentally, a substrate is at a ground potential GND.
Thus, since the voltage of 5V is applied between the diffusion regions D1 and D2, a current flows from the diffusion region D2 to the diffusion region D1. Some of the electrons that flow from the diffusion region D1 to the diffusion region D2 through the substrate are attracted to the control gate CGR to which the high voltage (6V) is applied, and stored in the corresponding ONO film of the memory element MR as CHE (Channel Hot Electrons).
On the other hand, when the contents of the memory element MR are read, the potential of the diffusion region D1 remains at 0V and the potential of the diffusion region D2 is set to 1.5V. Further, the potential of the control gate CGL is set to 5V and the potential of each of the control gate CGR and word gate WG is set to 2.5V. Incidentally, the substrate is at the ground potential GND. In this state, the contents of the memory element MR are determined according to whether the current flows through the control gate CGR. That is, if the electrons are being stored in the memory element MR, then no current flows. If not so, then the current flows.
Thus, since the twin MONOS cell makes use of such an SSI (Source Side Injection) system that a high voltage is applied to one of two gates arranged in a lateral direction with respect to a channel to bring one to a strong inverse state and bring the other thereof to a weak inverse state, thereby generating a high electric field between the gates to carry out an electron injection, it has the feature that the efficiency of the electron injection is high and high-speed writing can be performed with low power consumption. Since one memory cell MC has the two memory elements ML and MR, the twin MONOS cell has the feature that its densification is enabled.
However, the twin MONOS cell has the following problems.
That is, since it is necessary to form the gates on the ONO films, the structure of the memory cell becomes complicated and strict alignment becomes necessary, whereby Its manufacture becomes difficult. Since the diffusion regions D are formed on the substrate side-by-side so as to adjoin each other, a problem arises in that there is a fear that the influence of a high voltage on each memory element unintended for writing occurs.